A synthesizer is an electronic musical instrument that produces electronically generated sound. It is used to alter or manipulate already existing sounds to voices or other musical instruments and musical arrangements. It creates sounds by direct manipulation of electrical voltages, by the use of computers and manipulation of discrete values, or a combination of both. It uses subtractive, additive, physical modeling synthesis, FM, and phase distortion techniques to alter sound. It typically comes with a keyboard that provides the human interface for controlling. The keys can be in the form of piano keys, knobs, fingerboards or switchboards, and ribbons. Depending on the model, a synthesizer can have features such as delay, echo, reverb, phaser, distortion, volume, filters, multiple layers, and MIDI setup.